Thanks for your quick response.
I tried using the .setvalue.
I could never it it to work becuase it said the Xpathnavigator did not accept that data type. I tried this using double, int, and string
eventually i figured out how to do this calculation using the formual box in code.
I'm still curious why i could not save back to the form. I will have to figure this out for the next form I am going to develop,